Conventionally, when an image signal, picked up in a progressive image format of 24 frames/second (hereinafter, this image signal is referred to as a 24p image signal), is recorded as an image signal in an interlace image format of 60 fields/second (hereinafter, this image signal is referred to as a 60i image signal), a conversion process referred to as a 2:3:2:3 pull-down system is generally carried out.
FIG. 14 schematically shows the process of the 2:3:2:3 pull-down system. In FIG. 14, frame data corresponding to consecutive 4 frames of 24p image signals are indicated as A, B, C and D. FIG. 14 shows the process in which the 60i image signal corresponding to these 4 frames is converted with the time axis thereof being adjusted. In these processes, frame data A of a 24p image signal are separated into field data (Ao) consisting of odd lines of the 60i image signals and field data (Ae) consisting of even lines thereof. These field data (Ao) and (Ae) are respectively recorded on areas of field numbers 1 and 2 of the 60i image signal. Successively, in the same manner, frame data b of the 24p image signal are recorded on areas of field numbers 3, 4 and 5 of the 60i image signal. Frame data C of the 24p image signal are recorded on areas of field numbers 6 and 7 of the 60i image signal. Frame data d of the 24p image signal are recorded on areas of field numbers 8, 9 and 10 of the 60i image signal. Thereafter, the above-mentioned processes (processes in which frame data corresponding to 4 frames of the 24p image signal are converted to field data, and then recorded on 10 fields of the 60i image signal in a partially overlapped state of 2:3:2:3) are carried out in a cycle of 4 frames of the 24p image signal. This conversion process corresponds to the 2:3:2:3 pull-down conversion process.
In recent years, techniques in which images are compressed on a frame basis and recorded have been widely used. In the above-mentioned 2:3:2:3 pull-down conversion process, when a compression process is further carried out, the process is carried out in the following manner. Here, the compression process is explained by exemplifying a case in which frame data A of frame number 1 of a 60i image signal shown in FIG. 14 are compressed. In this case, field data (Ao) and (Ae) of the 60i image signal that have been subjected to the 2:3:2:3 pull-down conversion process are once combined into frame data A, and then subjected to a compression process. Hereinafter, the 60i image signal which has been subjected to the 2:3:2:3 pull-down conversion process and compressed in this manner is referred to as the compressed 60i image signal (2:3).
Upon reproducing and editing, frame data as picked up (frame data of a 24p image signal) are extracted from the compressed 60i image signal (2:3). More specifically, the compressed 60i image signal (2:3) is inversely converted to a 24p image signal in a compressed state (hereinafter, referred to as the compressed 24p image signal). In this case, the frame data of the 24p image signal in its compressed state are extracted in order to reduce the amount of data and to prevent degradation in the image.
In this inverse conversion process, frame data A, B and D of the compressed 24p image signal are inversely converted from the field data stored in areas of frame numbers, 1, 2 and 5 of the compressed 60i image signal (2:3). Frame data C of the compressed 24p image signal are inversely converted from the field data stored in areas of frame numbers 3 and 4 of the compressed 60i image signal (2:3).
In this inverse conversion process, for example, field data (Ao) and field data (Ae) corresponding to frame A (frame number 1) of the compressed 60i image signal are first combined to frame data A, and then compressed. With this arrangement, the field data (Ao) and (Ae), extracted from the frame number 1 of the compressed 60i image signal (2:3) form frame data A of the compressed 24p image signal. In the same manner, the field data (Bo, Be) and (Do, De), extracted from the frame numbers 2 and 5 of the compressed 60i image signal (2:3) form frame data b, D of the compressed 24p image signal.
However, in the case when field data (Bo, Ce) and (Ce, Do) are extracted from areas of frame numbers 3 and 4 of the compressed 60i image signal (2:3) to form frame data C of the compressed 24p image signal, it is not possible to apply the above-mentioned inverse conversion process. In this case, compressed field data (Bo, Ce) and (Co, De) constituting two frames of frame number 3 and frame number 4 are respectively expanded. Hereinafter, the field data that have been expanded are referred to as the expanded field data. Moreover, frame data to be combined from the expanded field data are referred to as expanded frame data.
After the above-mentioned processes, expanded frame data C are composed from expanded field data (Ce) (field number 6) and expanded field data (Co) (field number 7). Then, the expanded frame data C are again compressed to form compressed field data C. Since the inverse conversion process is required to execute such operations, the corresponding processes takes a long time, and the repeated compressing and expanding processes inevitably cause degradation in the image quality.
The objective of the present invention is to eliminate the necessity of the above-mentioned compressing/expanding processes that are required upon carrying out inverse conversion, to properly maintain the image quality and to improve the processing rate.